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Date: Tue, 13 Sep 2016 09:12:29 -0400 (EDT)
From: Alan Stern <stern@rowland.harvard.edu>
X-X-Sender: stern@netrider.rowland.org
To: Arnd Bergmann <arnd@arndb.de>
cc: Roger Quadros <rogerq@ti.com>, <gregkh@linuxfoundation.org>,
        <ssantosh@kernel.org>, <grygorii.strashko@ti.com>,
        <linux-usb@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4] usb: core: setup dma_pfn_offset for USB devices and,
 interfaces
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On Tue, 13 Sep 2016, Arnd Bergmann wrote:

> On Tuesday, September 13, 2016 11:16:03 AM CEST Roger Quadros wrote:
> > If dma_pfn_offset is not inherited correctly from the host controller,
> > it might result in sub-optimal configuration as bounce
> > buffer limit might be set to less than optimal level.
> > 
> > Consider the mass storage device case.
> > USB storage driver creates a scsi host for the mass storage interface in
> > drivers/usb/storage/usb.c
> > The scsi host parent device is nothing but the the USB interface device.
> > Now, __scsi_init_queue() calls scsi_calculate_bounce_limit() to find out
> > and set the block layer bounce limit.
> > scsi_calculate_bounce_limit() uses dma_max_pfn(host_dev) to get the
> > bounce_limit. host_dev is nothing but the device representing the
> > mass storage interface.
> > If that device doesn't have the right dma_pfn_offset, then dma_max_pfn()
> > is messed up and the bounce buffer limit is wrong.
> > 
> > e.g. On Keystone 2 systems, dma_max_pfn() is 0x87FFFF and dma_mask_pfn
> > is 0xFFFFF. Consider a mass storage use case: Without this patch,
> > usb scsi host device (usb-storage) will get a dma_pfn_offset of 0 resulting
> > in a dma_max_pfn() of 0xFFFFF within the scsi layer
> > (scsi_calculate_bounce_limit()).
> > This will result in bounce buffers being unnecessarily used.
> > 
> > Hint: On 32-bit ARM platforms dma_max_pfn() = dma_mask_pfn + dma_pfn_offset
> > 
> > Signed-off-by: Roger Quadros <rogerq@ti.com>
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>

Acked-by: Alan Stern <stern@rowland.harvard.edu>